Graphic documents comprise graphic elements. The elements may describe text, artwork and image information for example. Graphic documents can be produced by any of a wide a variety of document creation applications. Color matching refers to transforming graphic element color so that, when a graphic element is displayed on a target device, the color produced by the target device is consistent with an expected color.
A graphic document may be produced by a variety of artists contributing different parts of the document. Each artist may define color using different color models (e.g. specific to a device or device independent). Thus, different graphic elements in a finished work may define color in different ways.
Graphic elements may be defined to overlap. This may result from the artist's intent (e.g. text printing over an image or artwork). When this occurs the artist determines the order in which the overlapping graphic elements paint. An artist may determine that a graphic element obscures part of another graphic element that paints earlier in a region where they overlap. This is accomplished by disabling the graphic element's overprint attributes.
Alternatively the artist can enable overprint, which allows a graphic element that paints earlier in an overlap region to show some of its color through the graphic element enabling overprint. If several graphic elements overlap in a region and all but the first one enable overprinting then each graphic element, including the one that painted first, may show some of their color in the overlap region. The overprint setting for the first graphic element does not matter since it is not on top of any other graphic elements.
Sometimes overprints occur independently of the artist's intent. Preparation of a document for printing can involve a variety of processes, including automated processes, that can alter a document. Trap processing is an example of a process that adds graphic elements to a document to improve its quality under certain printing conditions. A trap processor may intentionally introduce small overprint-enabled graphic elements along boundaries between adjacent or overlapping graphic elements.
The meaning of overprint attribute settings is defined by an imaging model supported by an image processor. Adobe® PostScript® and Portable Document Format (PDF) are examples of page definition languages (PDLs) supported by raster image processors. These PDLs define opaque and transparent imaging models that include color contribution rules that determine how colors from layered graphic elements will display. A raster image processor (RIP) interprets a PDL to paint graphic elements as pixels, taking painting order into account. At a pixel location where graphic elements overlap, the RIP uses color contribution rules to determine the color of the pixel, based on the color of each pixel corresponding to an overlapping graphic element.
A RIP produces raster for a specific device and thus transforms color defined for a different device or independent of a device to a model that is suitable for the specific device during the process of pixel creation. The transformation is based on information that correlates the different color models. Color matching is a similar process of transforming color with the intent that color displayed on a specific device will meet some expectation. An expected color can be based on, for example, a reference color chart, measured color values, or the color produced by a specific device.
It is convenient to perform color matching during or after the creation of pixels by an image processor since the image processor typically incorporates the overprint color contribution rules. However, the color matching process is computationally intensive and in some situations, any latency introduced while producing raster for the printing device is not desirable. Additionally, honoring the color information (e.g. color profile) associated with each graphic element may not be possible in some raster image processors.
One approach to addressing this latency is to perform color matching before delivering the document to the image processor. In one prior art example of this approach, the color matching engine ignored any overlapping graphic elements. The reason for this was that matching the color of overlapping graphic elements would result in the image processor creating an undesirable color in the overlap region. Although the non-overlapped colors would be correct, the minor adjustments in colors would cause the overprint color contribution rules to dictate a significantly different color than if the color matching had not occurred.
Another approach is to incorporate color contribution rules into a color matching engine so that only those overlapping graphic elements that would contribute color to an overlap region are ignored by the color matching engine.
Another approach that could be used to provide compatibility with image processors that do not support overprint color contribution rules, involves altering documents to eliminate overlaps. Overlaps can be eliminated by “flattening” the graphic elements. In overlap regions, the original graphic elements can be clipped out of the region and a new graphic element added to represent the overlap region. The color of the new graphic element can be based on a narrow gamut color model including only cyan, magenta, yellow and black colorants. Thus, processing a flattened document using historical color matching tools would result in expected color in all regions. One disadvantage of this approach is that it requires altering the graphic elements created by the artist or added by a print production process. It is convenient to edit at the graphic element level. This becomes much more difficult if the graphic elements have been altered. Another disadvantage of this approach is that the color model chosen for an added graphic element can result in some loss of color fidelity if the fidelity of the contributing graphic element color and the target device is higher than the color model chosen to represent added graphic elements.
Another approach involves altering a document to paint a new graphic element on top of each overlap region. Each new graphic element may be added with overprint disabled so that it is the only element that contributes color to the overlap region. The representation chosen for the new graphic element is raster. One disadvantage of this approach is that raster is not always the best representation for any combination of overlapped graphic elements. The accuracy of the new graphic element's color and geometry may be limited. For example, if the display device is not a raster device converting a vector graphic element into a raster graphic element results in some unnecessary quantization. Similarly, if the target raster display device has a resolution that is much higher than assumed when creating the new raster graphic element, the graphic element color and geometry may be too coarse. In addition, color fidelity for raster graphic elements can be limited to eight bits per color component in some embodiments. Another disadvantage is that creation of raster can be a computationally intensive process.
There is a need for a color matching solution that can produce a color matched document with correct handling of overlap regions that preserves the integrity of the original document. Ideally such a solution would allow convenient editing of the color matched document. It is also desirable to preserve the accuracy and fidelity of the original document without adding unnecessary complexity.